Publications

Export 96 results:
Sort by: Author [ Title  (Asc)] Type Year
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 
M
Agnew, DC.  2001.  Map Projections to show the possible effects of surface loading. Journal of the geodetic Society of Japan. 47:255-260. Abstract
n/a
Johnson, HO, Agnew DC.  1995.  Monument Motion and Measurements of Crustal Velocities. Geophysical Research Letters. 22:2905-2908.   10.1029/95gl02661   AbstractWebsite

It is usually assumed in geodetic studies that measurement errors are independent from one measurement to the next and that the rate of deformation (velocity) is constant over the duration of the experiment. Any temporal correlation between measurements can substantially affect the uncertainty in this velocity estimate when it is determined;from the time series of measurements. One source of possible long-term. correlation is motion of the geodetic monument with respect to the ''deep'' crust. Available measurements suggest that this motion introduces errors that have the form of a random walk process. We show how such errors affect the uncertainty of velocity estimates. For a geodetic experiment of set duration we calculate the velocity uncertainty as a function of the number of observations and of the relative amount of correlated and uncorrelated noise. We find that 1) neglecting long-term temporal correlations makes the uncertainty in the estimated velocities much too small, and that 2) when the correlated and independent noise sources are of similar magnitude, the expected improvement in uncertainty from having more measurements (1/root N) is not realized; there is almost no improvement in some cases. We have also examined the effect of outliers (''blunders'') on the velocity uncertainty; for a frequency of outliers typical of geodetic field the previous two conclusions remain These results suggest that long-term correlations have a large effect on estimating deformation rates; unless these correlations are small, frequent observations give little advantage. If frequent observations are planned, the amount of correlated noise due to monument instability must be kept small if the full capabilities of the measurement technique are to be realized.

N
Sandwell, DT, Sichoix L, Agnew D, Bock Y, Minster JB.  2000.  Near real-time radar interferometry of the Mw 7.1 Hector Mine Earthquake. Geophysical Research Letters. 27:3101-3104.   10.1029/1999gl011209   AbstractWebsite

The Hector Mine Earthquake (Mw 7.1, 16 October 1999) ruptured 45 km of previously mapped and unmapped faults in the Mojave Desert. The ERS-2 satellite imaged the Mojave Desert on 15 September and again on 20 October, just 4 days after the earthquake. Using a newly-developed ground station we acquired both passes and were able to form an interferogram within 20 hours of the second overflight. Estimates of slip along the main rupture are 1-2 meters greater than slip derived from geological mapping. The gradient of the interferometric phase reveals an interesting pattern of triggered slip on adjacent faults as well as a 30 mm deep sink hole along Interstate 40.

Agnew, DC.  1997.  NLOADF: A program for computing ocean-tide loading. Journal of Geophysical Research-Solid Earth. 102:5109-5110.   10.1029/96jb03458   AbstractWebsite

The loading of the Earth by the ocean tides produces several kinds of signals which can be measured by geodetic technique. In order to compute these most accurately; a combination of global and local models of the ocean tides may be needed. The program NLOADF convolves the Green functions for loading with ocean tide models using a station-centered grid with fixed dimensions, making it easy to combine different ocean models without overlap in the convolution. The program computes all the quantities of interest (gravity, displacement, tilt, and strain) and includes the case where measurements are made beneath the surface of the ocean.

Barbour, AJ, Agnew DC.  2012.  Noise Levels on Plate Boundary Observatory Borehole Strainmeters in Southern California. Bulletin of the Seismological Society of America. 101:2453-2466.   10.1785/0120110062   AbstractWebsite

To establish noise levels for the borehole strainmeters of the Plate Boundary Observatory (PBO), we have analyzed data recorded by eight of these instruments, all in the Anza region of southern California. We determine time-varying power spectra for frequencies from 10(-3) to 10 Hz, using a new method that combines multitaper spectrum estimation, smoothing by local regression, and computation of cumulative distribution functions. From about 2 Hz to the Nyquist frequency of 10 Hz, the noise floor is set by instrument resolution; for frequencies between 0.1 Hz and 1 Hz, it is set by microseisms. The lowest noise level is between 0.01 and 0.1 Hz, with a rapid increase at lower frequencies. However, in most instruments this low-noise range also contains narrowband noise that appears to be caused by power supply fluctuations. We compare these results with noise spectra from other types of strainmeters, which suggest two conclusions: (1) they are in agreement with results for surficial, long-baseline instruments; and (2) other subsurface strainmeters have lower noise in the seismic band than the PBO instruments do.

Agnew, DC.  1981.  Nonlinearity in Rock - Evidence from Earth Tide. Journal of Geophysical Research. 86:3969-3978.   10.1029/JB086iB05p03969   AbstractWebsite

The earth is sinusoidally stressed by tidal forces; if the stress-strain relation for rock is nonlinear, energy should appear in an earth tide record at frequencies which are multiples of those of the larger tidal lines. An examination of the signals to be expected for different nonlinear deformation laws shows that for a nonlinear response without dissipation, the largest anomalous signal should occur at twice the forcing frequency, whereas for nonlinear laws involving dissipation (cusped hysteresis loops) the anomalous signal will be greatest at 3 times this frequency. The size of the signal in the dissipative case depends on the amount by which dissipation affects the particular response being measured. For measurements of strain tides this depends on whether dissipation is assumed to be present throughout the earth or localized around the point of measurement. An analysis of 5.7 years of strain tide records from Piñon Flat, California, shows a small signal at twice the frequency of the largest (M2) tide. Most of the observed signal can be explained by loading from nonlinear water tides in the Gulf of California and the Pacific Ocean; the residual nonlinear tide is 65 dB less than the M2 tide. The signal at 3 times the M2 frequency is compatible with a linear model or with nonlinear hysteresis loops provided that nonlinear dissipation occurs throughout the earth. Nonlinear dissipation in the rocks near the strainmeter would produce a larger signal than is seen.

O
Davis, PM, Rydelek PA, Agnew DC, Okamura AT.  1987.  Observation of Tidal Tilt on Kilauea Volcano, Hawaii. Geophysical Journal of the Royal Astronomical Society. 90:233-244.   10.1111/j.1365-246X.1987.tb00682.x   AbstractWebsite

We have analysed the east-west tilt components, O1, K1, N2, M2 and S2 from a continuously recording tiltmeter located in Uwekahuna Vault on Kilauea Volcano, Hawaii, for the period 1971—79. Detailed analysis of the M2 component gives values of 30.9 ± 2.0 (95 per cent) nrad and 116.0 ± 2.0° for the amplitude and phase, respectively, compared to values of 48.5 nrad and 139.4° for the equilibrium tide. the total theoretical tide, found by summing the equilibrium and load tides, amounts to 37.2 nrad at a phase of 121.7°. the 20 per cent discrepancy with that observed may be due to an inaccurate cotical chart, cavity effects in the vault, strain—tilt coupling or an inappropriate solid earth model. In the vicinity of Hawaii (≤ 3°) two independent cotidal charts give almost identical results for the near field ocean load. At greater distances, we use the Schwiderski (1978) cotidal chart. We estimate that local cavity and strain—tilt coupling effects are less than 12 per cent owing to the agreement between geodetically determined and instrumental tilt but we can not rule out regional effects. Assuming these are small and the cotical charts correct, we find that the M2 results are brought into satisfactory agreement if, instead of using an average oceanic earth model in the (< 75 km) vicinity of Hawaii, we use an earth model with nearly one-half the oceanic rigidity. Such a low upper mantle and crustal rigidity is consistent with Kilauea's position above the thermal upwelling associated with the Hawaiian hotspot.

Knopoff, L, Rydelek PA, Zurn W, Agnew DC.  1989.  Observations of Load Tides at the South-Pole. Physics of the Earth and Planetary Interiors. 54:33-37.   10.1016/0031-9201(89)90184-2   AbstractWebsite

The use of tidal observations to study the ‘nearly diurnal free wobble’ mode of the Earth is possible if oceanic effects in the tidal record are accurately removed. We have analyzed vertical gravity data from the South Pole to determine the amplitude and phase of the small daily and semidaily tides observed at the Pole. Since these signals at the Pole are most probably caused by oceanic tides, our observations provide an excellent target for the oceanic models. A comparison with the best models of the oceans now available indicates the need for improvement in measuring and modeling the southern oceans.

Agnew, DC.  1995.  Ocean-Load Tides at the South-Pole - A Validation fo Recent Ocean-Tide Models. Geophysical Research Letters. 22:3063-3066.   10.1029/95gl03074   AbstractWebsite

Small diurnal and semidiurnal gravity tides are seen at the South Pole because of the loading by and attraction of the ocean tides. These data provide a check on the quality of ocean-tide models, especially in the southernmost ocean, which has historically been the most lacking in tidal data. Ocean-tide models developed in the 1980's did not predict the gravity tides at this location very well. Recently-developed models based on the Topex/Poseidon altimetric data and improved hydrodynamical modeling agree much better with the observations, provided that the tides beneath the ice shelves are included. The level of agreement at this remote location suggests that, loads from very local tides aside, the new generation of ocean-tide models can predict the loading tides to very high accuracy.

Borsa, AA, Agnew DC, Cayan DR.  2014.  Ongoing drought-induced uplift in the western United States. Science.   10.1126/science.1260279   AbstractWebsite

The western United States has been experiencing severe drought since 2013. The solid earth response to the accompanying loss of surface and near-surface water mass should be a broad region of uplift. We use seasonally-adjusted time series from continuously operating GPS stations to measure this uplift, which we invert to estimate mass loss. The median uplift is 4 mm, with values up to 15 mm in California’s mountains. The associated pattern of mass loss, which ranges up to 50 cm of water equivalent, is consistent with observed decreases in precipitation and streamflow. We estimate the total deficit to be about 240 Gt, equivalent to a 10 cm layer of water over the entire region, or the annual mass loss from the Greenland Ice Sheet.

P
Anderson, JG, Rockwell TK, Agnew DC.  1989.  Past and possible future earthquakes of significance to the San Diego region. Earthquake Spectra. 5:299-335. Abstract
n/a
Astiz, L, Shearer PM, Agnew DC.  2000.  Precise relocations and stress change calculations for the upland earthquake sequence in southern California. Journal of Geophysical Research-Solid Earth. 105:2937-2953.   10.1029/1999jb900336   AbstractWebsite

We relocate earthquakes that occurred near the 1988 (M-L = 4.7) and the 1990 (M-L = 5.5) Upland, California, earthquakes to map the fault geometry of the poorly defined San Jose fault and to test the static:Stress triggering hypothesis for this sequence. We adopt the L1 norm, waveform cross-correlation method of Shearer [1997] to obtain precise relocations for 1573 events: between 1981 and 1997 in the Upland area. To limit computation time, we only perform waveform cross correlation on 60 of the nearest neighbors of leach relocated event. Our final relocations show two linear features. The first is imaged,by the locations of the initial month of aftershocks of the 1988 Upland earthquake, which delineate a fault with a,dip angle of similar to 45 degrees between 7 and 9 km depth, consistent with the mainshock focal mechanism. The second linear feature is a plane, dipping at about 74 degrees from 2 to 9 km depth, which is illuminated by both the 1988:and 1990 Upland sequences, in agreement with the inferred location of the San Jose fault at depth. However, below 9 km the event locations become more diffuse, giving rise to two different interpretations of the fate of the San Jose fault at depth. One possibility is that the fault shallows at depth, consistent with our relocations: but not with the focal mechanism of a M-L = 4.7 deep aftershock. Alternatively, the. fault may be offset at depth by the more shallow dipping fault strand broken during the 1988 earthquake, Using these inferred fault geometries, we compute stress changes resulting from slip during the mainshocks to test whether the relocated aftershocks are consistent with the:hypothesis that more aftershocks occur where the change in static Coulomb failure stress is positive (on faults optimally oriented for failure). This requires an extension of previous models of changes in the failure stress to three dimensions and arbitrary fault orientation. We find that patterns of change in Coulomb failure stress differ little between the different fault geometries: all are nearly symmetric about the fault and so do not match the aftershock distribution, in which most of the off-fault events occur to one side of the fault plane.

Agnew, DC, Jones LM.  1991.  Prediction Probabilities from Foreshocks. Journal of Geophysical Research-Solid Earth and Planets. 96:11959-11971.   10.1029/91jb00191   AbstractWebsite

When any earthquake occurs, the possibility that it might be a foreshock increases the probability that a larger earthquake will occur nearby within the next few days. Clearly, the probability of a very large earthquake ought to be higher if the candidate foreshock were on or near a fault capable of producing that very large mainshock, especially if the fault is towards the end of its seismic cycle. We derive an expression for the probability of a major earthquake characteristic to a particular fault segment, given the occurrence of a potential foreshock near the fault. To evaluate this expression, we need: (1) the rate of background seismic activity in the area, (2) the long-term probability of a large earthquake on the fault, and (3) the rate at which foreshocks precede large earthquakes, as a function of time, magnitude, and spatial location. For this last function we assume the average properties of foreshocks to moderate earthquakes in California: (1) the rate of mainshock occurrence after foreshocks decays roughly as t-1, so that most foreshocks are within three days of their mainshock, (2) foreshocks and mainshocks occur within 10 km of each other, and (3) the fraction of mainshocks with foreshocks increases linearly as the magnitude threshold for foreshocks decreases, with 50% of the mainshocks having foreshocks with magnitudes within three units of the mainshock magnitude (within three days). We apply our results to the San Andreas, Hayward, San Jacinto, and Imperial faults, using the probabilities of large earthquakes from the report of the Working Group on California Earthquake Probabilities (1988). The magnitude of candidate event required to produce a 1% probability of a large earthquake on the San Andreas fault within three days ranges from a high of 5.3 for the segment in San Gorgonio Pass to a low of 3.6 for the Carrizo Plain.

Johnson, HO, Agnew DC, Wyatt FK.  1994.  Present-Day Crustal Deformation in Southern California. Journal of Geophysical Research-Solid Earth. 99:23951-23974.   10.1029/94jb01902   AbstractWebsite

The effects of laterally homogeneous mantle electrical conductivity have been included in steady. Using an extensive set of precise geodetic measurements, we have developed a detailed picture of present-day deformation rates in southern California. This large set of measurements, amounting to nearly 2000 repeated distance measurements over the period 1973 to 1991, comes from the U.S. Geological Survey's Geodolite trilateration program, involving their combined Anza, Joshua Tree, and Salton networks. Building on previous results from these data, we are able to present the deformation field as estimates of the rate of horizontal strain accumulation in small four-station subnetworks of the overall 89-station network.; Using this technique, the spatial details of the 18-year average strain rate field can be determined. By correlating these spatial details with the tectonics of the region we are able to understand better how deformation is partitioned across this highly complex margin between the Pacific and North American tectonic plates. Some of the more interesting findings of this study are that (1) the vast majority of strain rate estimates show a pattern of nearly pure shear as would be expected in a transcurrent environment, (2) the fastest accumulation of surface strain in southern California is along the San Jacinto Fault west of the Salton Sea, not along the San Andreas Fault, (3) strain accumulation rate along the length of the San Jacinto Fault increases toward the southeast as the fault enters the Imperial Valley, (4) a large area near the southern end of the Salton Sea, where the San Andreas Fault meets the Brawley Seismic Zone, is undergoing areal dilatation, which is in part consistent with the formation of crust at a spreading center, and (5) deformation at the transition zone between the San Andreas Fault and the Eastern California Shear Zone also appears to be the result of crustal spreading.

Agnew, DC, Berger J, Farrell WE, Gilbert JF, Masters G, Miller D.  1986.  Project IDA: a decade in review. EOS Trans. AGU. 67:203-212. Abstract
n/a
R
Agnew, DC.  2013.  Realistic Simulations of Geodetic Network Data: The Fakenet Package. Seismological Research Letters. 84:426-432.   10.1785/0220120185   AbstractWebsite
n/a
Gomberg, J, Wech A, Creager K, Obara K, Agnew D.  2016.  Reconsidering earthquake scaling. Geophysical Research Letters. 43:6243-6251.   10.1002/2016gl069967   AbstractWebsite

The relationship (scaling) between scalar moment, M-0, and duration, T, potentially provides key constraints on the physics governing fault slip. The prevailing interpretation of M-0-T observations proposes different scaling for fast (earthquakes) and slow (mostly aseismic) slip populations and thus fundamentally different driving mechanisms. We show that a single model of slip events within bounded slip zones may explain nearly all fast and slow slip M-0-T observations, and both slip populations have a change in scaling, where the slip area growth changes from 2-D when too small to sense the boundaries to 1-D when large enough to be bounded. We present new fast and slow slip M-0-T observations that sample the change in scaling in each population, which are consistent with our interpretation. We suggest that a continuous but bimodal distribution of slip modes exists and M-0-T observations alone may not imply a fundamental difference between fast and slow slip.

Rolandone, F, Burgmann R, Agnew DC, Johanson IA, Templeton DC, d'Alessio MA, Titus SJ, DeMets C, Tikoff B.  2009.  Reply to comment by J. C. Savage on "Aseismic slip and fault-normal strain along the creeping section of the San Andreas Fault''. Geophysical Research Letters. 36   10.1029/2009gl039167   AbstractWebsite
n/a
Agnew, DC.  2004.  Robert Fitzroy and the myth of the 'Marsden Square': Transatlantic rivalries in early marine meteorology. Notes and Records of the Royal Society of London. 58:21-46.   10.1098/rsnr.2003.0223   AbstractWebsite

Marine data (especially in meteorology) are often grouped geographically using a set of numbered 10degrees latitude-longitude squares known as Marsden squares, which are usually attributed to William Marsden, Secretary of the Admiralty (and Vice-President of The Royal Society), who supposedly invented them early in the nineteenth century. Available records suggest that this system was in fact probably invented by Robert FitzRoy soon after his appointment as head of the British Meteorological Office in 1854. FitzRoy felt that early English work in marine meteorology was being ignored, notably by the American Matthew Fontaine Maury, who had pioneered the collecting of marine meteorological data from ship's logs. A desire to undo this wrong led FitzRoy to emphasize earlier (though abortive) British projects by A.B. Becher (in 1831) and by Marsden (probably in the 1780s), both of which involved grouping marine data geographically, though only over limited areas. FitzRoy's treatment of this earlier work seems to have created, much later, the belief that Marsden had invented the system of 10degrees squares. Given both Maury's and FitzRoy's desire to demonstrate priority in this field, it is ironic that the first clear proposal to collect and group data from ship's logs was made by the American (and British) natural philosopher Isaac Greenwood in 1728.

Agnew, DC.  1989.  Robust Pilot Spectrum Estimation for the Quality-Control of Digital Seismic Data. Bulletin of the Seismological Society of America. 79:180-188. AbstractWebsite
n/a
S
Anderson, G, Agnew DC, Johnson HO.  2003.  Salton trough regional deformation estimated from combined trilateration and survey-mode GPS data. Bulletin of the Seismological Society of America. 93:2402-2414.   10.1785/0120030014   AbstractWebsite

The Salton Trough in southeastern California, United States, has one of the highest seismicity and deformation rates in southern California, including 20 earthquakes M 6 or larger since 1892. From 1972 through 1987, the U.S. Geological Survey (USGS) measured a 41-station trilateration network in this region. We remeasured 37 of the USGS baselines using survey-mode Global Positioning System methods from 1995 through 1999. We estimate the Salton Trough deformation field over a nearly 30-year period through combined analysis of baseline length time series from these two datasets. Our primary result is that strain accumulation has been steady over our observation span, at a resolution of about 0.05 mustrain/yr at 95% confidence, with no evidence for significant long-term strain transients despite the occurrence of seven large regional earthquakes during our observation period. Similar to earlier studies, we find that the regional strain field is consistent with 0.5 +/- 0.03 mustrain/yr total engineering shear strain along an axis oriented 311.6degrees +/- 23degrees east of north, approximately parallel to the strike of the major regional faults, the San Andreas and San Jacinto (all uncertainties in the text and tables are standard deviations unless otherwise noted). We also find that (1) the shear strain rate near the San Jacinto fault is at least as high as it is near the San Andreas fault, (2) the areal dilatation near the southeastern Salton Sea is significant, and (3) one station near the southeastern Salton Sea moved anomalously during the period 1987.95-1995.11.

Agnew, D.  1989.  Seismic instrumentation. The Encyclopedia of solid earth geophysics. ( James DE, Ed.).:1033-1037., New York: Van Nostrand Reinhold Abstract
n/a
Berger, J, Agnew DC, Parker RL, Farrell WE.  1979.  Seismic System Calibration 2. Cross-Spectral Calibration Using Random Binary Signals. Bulletin of the Seismological Society of America. 69:271-288. AbstractWebsite
n/a
Nikolaidis, RM, Bock Y, de Jonge PJ, Shearer P, Agnew DC, vanDomselaar M.  2001.  Seismic wave observations with the Global Positioning System. Journal of Geophysical Research-Solid Earth. 106:21897-21916.   10.1029/2001jb000329   AbstractWebsite

We describe the direct measurement of ground displacement caused by the Hector Mine earthquake in southern California (M-w 7.1, October 16, 1999). We use a new method of instantaneous positioning, which estimates site coordinates from only a single epoch of Global Positioning System (GPS) data, to measure dynamic as well as static displacements at 24 stations of the Southern California Integrated GPS Network (SCIGN), with epicentral distances from 50 to 200 km. For sites outside the Los Angeles basin the observed displacements are well predicted by an elastic half-space model with a point shear dislocation; within the sedimentary basin we observe large displacements with amplitudes up to several centimeters that last as long as 3-4 min. Since we resolve the GPS phase ambiguities and determine site coordinates independently at each epoch, the GPS solution rate is the same as the receiver sampling rate. For the SCIGN data this is 0.033 Hz (once per 30 s), though sample rates up to 2 Hz are possible with the SCIGN receivers. Since the GPS phase data are largely uncorrelated at I s, a higher sampling rate would offer improved temporal resolution of ground displacement, so that in combination with inertial seismic data, instantaneous GPS positioning would in many cases significantly increase the observable frequency band for strong ground motions.

Elkhoury, JE, Brodsky EE, Agnew DC.  2006.  Seismic waves increase permeability. Nature. 441:1135-1138.   10.1038/nature04798   AbstractWebsite

Earthquakes have been observed to affect hydrological systems in a variety of ways-water well levels can change dramatically, streams can become fuller and spring discharges can increase at the time of earthquakes(1-7). Distant earthquakes may even increase the permeability in faults(8). Most of these hydrological observations can be explained by some form of permeability increase(1,5). Here we use the response of water well levels to solid Earth tides to measure permeability over a 20-year period. At the time of each of seven earthquakes in Southern California, we observe transient changes of up to 24 degrees in the phase of the water level response to the dilatational volumetric strain of the semidiurnal tidal components of wells at the Pinon Flat Observatory in Southern California. After the earthquakes, the phase gradually returns to the background value at a rate of less than 0.1 degrees per day. We use a model of axisymmetric flow driven by an imposed head oscillation through a single, laterally extensive, confined, homogeneous and isotropic aquifer to relate the phase response to aquifer properties(9). We interpret the changes in phase response as due to changes in permeability. At the time of the earthquakes, the permeability at the site increases by a factor as high as three. The permeability increase depends roughly linearly on the amplitude of seismic-wave peak ground velocity in the range of 0.21-2.1 cm s(-1). Such permeability increases are of interest to hydrologists and oil reservoir engineers as they affect fluid flow and might determine long-term evolution of hydrological and oil-bearing systems. They may also be interesting to seismologists, as the resulting pore pressure changes can affect earthquakes by changing normal stresses on faults(10).